President Vladimir V. Putin of Russia has warned that if F-16 fighter jets supplied to Ukraine by its Western allies operated from airfields in other countries, the bases would be “legitimate targets” for attack.

In a speech to Russian Air Force pilots late Wednesday, however, Mr. Putin rejected suggestions from some Western leaders that Russia is planning to invade NATO countries as “complete nonsense.”

The threat that Russia might move against other countries has become one of the main arguments used by the Ukrainian government and its supporters to try to persuade the U.S. to dispatch more military aid to the country.

The Ukrainian president, Volodymyr Zelensky, said again in an interview with CBS News published on Thursday that war “can come to Europe, and to the United States of America.”

“It can come very quickly to Europe,” said Mr. Zelensky.

At the United Nations on Thursday, Russia vetoed a resolution that would have extended U.N. oversight of sanctions tied to North Korea’s nuclear program, prompting accusations from Western nations that it was acting to keep the pipeline of weaponry open from the North Koreans for use in Ukraine.

The Russian vote — a reversal from its past support for the monitoring program — drew condemnation from world leaders. In the United States, the White House national security spokesman, John Kirby, called it a “reckless action” that made clear the growing ties between Pyongyang and Moscow.

The vote came a day after Mr. Putin visited the provincial town of Torzhok in the Tver region north of Moscow, where he inspected a training center for Russian military pilots.

Speaking with the pilots, Mr. Putin said that Russia spent only a fraction on its military of what the United States does. “Considering this difference,” he said, concerns that Moscow would fight NATO are “nonsense.”

Mr. Putin also reiterated his argument that NATO’s enlargement in the wake of the Soviet collapse was the primary reason for his decision to attack Ukraine in 2022.

“Did we cross the ocean and approach U.S. borders?” he asked. “No, they are the ones who are closing in on us, and they have reached our borders.”

But Mr. Putin did leave open the door to an expanded battlefield when it came to the decision by Ukraine’s allies to equip it with American-made F-16 fighter jets.

“F16 aircraft can also carry nuclear weapons, and we will have to heed this while organizing our combat operations,” said Mr. Putin.

At the United Nations on Thursday, Russia vetoed a resolution that would have extended U.N. oversight of sanctions tied to North Korea’s nuclear program, prompting accusations from Western nations that it was acting to keep the pipeline of weaponry open from the North Koreans for use in Ukraine.

Ukraine has also figured large in the Kremlin’s response to the terrorist attack that killed at least 143 people last week at a concert hall outside Moscow.

Since shortly after the attack, Mr. Putin and other Russian officials, without evidence, have depicted Ukraine as the likely mastermind behind it. Mr. Putin eventually acknowledged that the assault had been “perpetrated by radical Islamists,” but stuck with the contention that Ukraine could have ordered it.

On Thursday, continuing in that vein, Russia’s investigative committee, the country’s equivalent to the F.B.I., said in a statement that it had “obtained evidence” that the attackers were “connected with Ukrainian nationalists,” without specifying what that proof was.

Investigators said that they had “confirmed data that the perpetrators of the terrorist attack received significant amounts of money and cryptocurrency from Ukraine, which were used in preparing the crime.” They also said that another person had been arrested in connection with the attack and would soon appear in court.

The Ukrainian government has denied any involvement in the assault. The United States and other Western governments have said repeatedly that the Islamic State — which itself has issued two claims of responsibility — was behind the assault and that Ukraine was not involved.

With 74 victims of the concert hall attack still hospitalized, Mr. Putin has yet to visit them or to go to the site of the attack. Dmitri S. Peskov, the Kremlin spokesman said on Thursday that Mr. Putin had been heavily involved in the rescue effort on the night of the attack, and that a visit to the site of the attack would hinder their work.

Emin Agalarov, an executive with the owner of the concert hall, the Crocus Group, on Thursday defended how his staff had handled the attack and its aftermath, including a fire in the hall, and the building’s compliance with fire safety and security requirements. Speaking on state television, Mr. Agalarov said that the fire and other emergency systems had worked normally and allowed thousands of people to evacuate the building.